1. Field of the Invention
The present invention relates to data communications systems and methods, and more particularly, to bus systems and methods.
2. Statement of the Problem
High-bandwidth busses are typically used to communicate between hosts and peripherals in applications such as computer networks. The bus interfaces used by hosts and peripherals often take different forms depending on the performance characteristics desired. For example, the Peripheral Component Interconnect (PCI) standard defines a high speed synchronous bus well suited for local bus communications between processors and peripherals using low-power CMOS devices over relatively short 32-bit and 64-bit wide data paths. Relatively streamlined in operation, PCI busses can reach transfer rates of up to 264 Mbytes per second on long burst transfers. In contrast, the Small Computer System Interface (SCSI) defined by ANSI X3.131 defines busses which may-operate over much longer lengths without a bus signal, an arrangement more suited for interfacing with peripheral devices such as disk drives. The PCI and SCSI bus standards are described in "Understanding I/O Subsystems," published by Adaptec Press, 1st edition, 1996.
Bus specifications often limit, among other things, the length of the bus and the number of devices that may be attached to the bus in order to maintain performance. For example, PCI and SCSI standards define limits on conductor length and number of devices. In order to increase the capacity of a bus such as a PCI bus, an expanded multi-layer bus structure may be used that includes a plurality of busses connected by high-speed bus bridges. This multi-layer structure can allow an increased number of devices to be interconnected while maintaining bus performance.
Although computer systems and networks may be expanded by the addition of bus bridges, it may be problematic to provide for such expansion. For example, in a personal computer (PC) it is often desirable to provide space for mounting a plurality of local bus peripherals, e.g., PCI agents such as sound cards, video cards, local area network (LAN) cards and the like. Because the PCI standard limits a PCI bus to ten unit loads, a single PCI bus typically can only support three to four expansion slots. PCI-PCI bridges may be used to provide increased capacity, but the numbers of agents provided in particular units may vary widely. Accordingly, it may not be cost effective to provide PCI-PCI bus bridges for bus capacity that may not be utilized by all potential users.